Call Girls Horamavu WhatsApp Number 7001035870 Meeting With Bangalore Escorts
c h a p t e r 4 third week of development.pptx
1. c h a p t e r 4
ThirdWeek of Development:
Trilaminar GermDisc
April11,
2023
Dr.Najma Mohamed 1
2. Third Week of Development:
Trilaminar Germ Disc:
The most characteristic event occurring during
the third week of gestation is gastrulation,
the process that establishes all three germ
layers
1.ectoderm
2. mesoderm and
3. endoderm
Gastrulation begins with formation of the
primitive streak on the surface of the epiblast.
April11,
2023
Dr.Najma Mohamed 2
3. cont…
At beginning the streak is vaguely defined but in a 15-to 16-day
embryo, it is clearly visible as a narrow groove with slightly
bulging regions on either side.
The cephalic end of the streak, the primitive node, consists of
a slightly elevated area surrounding the small primitive pit.
Then the cells of the epiblast migrate to the primitive streak
and the cells become flask shaped, This inward movement is
known as invagination.
April11,
2023
Dr.Najma Mohamed 3
7. Cont..
Once the cells have invaginated, some displace the hypoblast,
creating the embryonic endoderm, and others come to lie
between the epiblast and newly created endoderm to form
mesoderm.
Cells remaining in the epiblast then form ectoderm.
As more and more cells move between the epiblast and
hypoblast layers, they begin to spread laterally and cephalad.
April11,
2023
Dr.Najma Mohamed 7
8. Con..
Gradually they migrate beyond the margin of the disc and
establish contact with the extraembryonic mesoderm covering
the yolk sac and amnion.
In the cephalic direction, they pass on each side of the
prechordal plate.
The prechordal plate itself forms between the tip of the
notochord and the buccopharyngeal membrane .
April11,
2023
Dr.Najma Mohamed 8
9. Formation of the Notochord:
Prenotochordal cells invaginating in the primitive pit
move forward cephalad until they reach the prechordal
plate.
These prenotochordal cells become intercalated in the
hypoblast so that, for a short time, the midline of the
embryo consists of two cell layers that form the
notochordal canal.
April11,
2023
Dr.Najma Mohamed 9
10. At the point where the pit forms an indentationin the
epiblast, the neurenteric canal temporarily connects
the amniotic and yolk sac cavities.
The cloacal membrane is formed at the caudal end of
the embryonic disc.
April11,
2023
Dr.Najma Mohamed 10
11. Gradually the walls of the canal become flatten and
called notochordal plate
Soon after the flattening is reversed and notochordal
plate again become curved
As the hypoblast is replaced by endoderm cells moving
in at the streak, cells of the notochordal plate
proliferate and detach from the endoderm.
They then form a solid cord of cells, the definitive
notochord, which underlies the neural tube and serves
as the basis for the axial skeleton.
April11,
2023
Dr.Najma Mohamed 11
15. Because elongation of the notochord is a dynamic process,
the cranial end forms first, and caudal regions are added as
the primitive streak assumes a more caudal position.
The notochord and prenotochordal cells extend cranially to
the prechordal plate (an area just caudal to the
buccopharyngeal membrane) and caudally to the primitive
pit.
April11,
2023
Dr.Najma Mohamed 15
16. This membrane, which is similar in structure to the
buccopharyngeal membrane, consists of tightly
adherent ectoderm and endoderm cells with no
intervening mesoderm.
April11,
2023
Dr.Najma Mohamed 16
17. When the cloacal membrane appears, the posterior wall
of the yolk sac forms a small diverticulum that extends
into the connecting stalk.
This diverticulum, the allantoenteric diverticulum, or
allantois, appears around the 16th day of development.
April11,
2023
Dr.Najma Mohamed 17
18. Establishment of the Body
Axes:
Establishment of the body axes, anteroposterior,
dorsoventral,and left-right, takes place before and
during the period of gastrulation.
April11,
2023
Dr.Najma Mohamed 18
19. Fate Map Established During
Gastrulation:
Regions of the epiblast that migrate and ingress through the
primitive streak have been mapped and their ultimate fates
determined. eg:
cells that ingress through the cranial region of the node
become notochord;
April11,
2023
Dr.Najma Mohamed 19
20. those migrating at the lateral edges of the node and from
the cranial end of the streak become paraxial mesoderm;
cells migrating through the midstreak region become
intermediate mesoderm;
those migrating through the more caudal part of the
streak form lateral plate mesoderm; and
cells migrating through the caudal-most part of the streak
contribute to extraembryonic mesoderm.
April11,
2023
Dr.Najma Mohamed 20
21. Growth of the Embryonic
Disc:
The embryonic disc, initially flat and almost round,
gradually becomes elongated, with a broad cephalic and
a narrow caudal end.
Expansion of the embryonic disc occurs mainly in the
cephalic region; the region of the primitive streak
remains more or less the same size.
April11,
2023
Dr.Najma Mohamed 21
22. Con..
Growth and elongation of the cephalic part of the disc
are caused by a continuous migration of cells from the
primitive streak region in a cephalic direction.
Invagination of surface cells in the primitive streak and
their subsequent migration forward and laterally
continues until the end of the fourth week.
April11,
2023
Dr.Najma Mohamed 22
23. At that stage, the primitive streak shows regressive
changes, rapidly shrinks, and soon disappears.
In the cephalic part, germ layers begin their specific
differentiation by the middle of the third week,
April11,
2023
Dr.Najma Mohamed 23
24. Cont..
Whereas in the caudal part, differentiation begins by
the end of the fourth week.
Thus gastrulation, or formation of the germ layers,
continues in caudal segments while cranial structures
are differentiating, causing the embryo to develop
cephalocaudally.
April11,
2023
Dr.Najma Mohamed 24
25. Further Development of the
Trophoblast:
By the beginning of the third week, the trophoblast is
characterized by primary villi that consist of a
cytotrophoblastic core covered by a syncytial layer.
During further development, mesodermal cells penetrate
the core of primary villi and grow toward the decidua.
The newly formed structure is known as a secondary villus.
April11,
2023
Dr.Najma Mohamed 25
28. Cont..
By the end of the third week, mesodermal cells in the
core of the villus begin to differentiate into blood cells
and small blood vessels, forming the villous capillary
system.
The villus is now known as a tertiary villusor definitive
placental villus.
April11,
2023
Dr.Najma Mohamed 28
30. Cont..
Capillaries in tertiary villi make contact with capillaries
developing in mesoderm of the chorionic plate and in the
connecting stalk.
These vessels, in turn, establish contact with the
intraembryonic circulatory system, connecting the placenta and
the embryo.
When the heart begins to beat in the fourth week of
development, the villous system is ready to supply the embryo
proper with essential nutrients and oxygen.
April11,
2023
Dr.Najma Mohamed 30
33. Con..
Meanwhile, cytotrophoblastic cells in the villi penetrate progressively into
the overlying syncytium until they reach the maternal endometrium.
Here they establish contact with similar extensions of neighboring villous
stems, forming a thin outer cytotrophoblast shell, which attaches the
chronic sac to the endometrium.
April11,
2023
Dr.Najma Mohamed 33
34. Villi that extend from the chorionic plate to the decidua
basalis (decidual plate:the part of the endometrium
where the placenta will form; are called stem or
anchoring villi.
April11,
2023
Dr.Najma Mohamed 34
